1. Field of the Invention
This invention relates to semiconductor devices, Micro Electro Mechanical Systems (MEMS), sensors and more specifically to three dimensional (3D) three-axis accelerometers, vibration sensors and inclinometers for consumer and other applications.
2. Description of the Related Art
MEMS accelerometers are known for more than 25 years and they are widely used in different areas. Automotive air-bag applications currently represent the biggest MEMS accelerometer market. Furthermore, there are only few known MEMS accelerometers that can measure all three components of an acceleration vector, three-axis accelerometers and 3D accelerometers.
The market for 3D accelerometers includes hand-held devices, cell phones, PDAs, hand-held computers, gaming devices, remote controls, etc., health and sport products including ergometers, smart shoes, patient posture indicators, pacemakers, biometric devices and systems, etc., monitoring systems for civil projects such as bridges, buildings, etc., smart toys, virtual reality devices, and more. However, these markets require low-cost, stable and reliable three-axis accelerometers, which have impeded market growth because of their high cost. The cost of 3D accelerometers can be dramatically reduced Therefore, there is a need for low-cost single die 3D accelerometer that possesses all the above-mentioned features.
FIG. 1 illustrates the principle and shows a structure of a three-axis accelerometer die with an elastic element in the form of uniform diaphragm 14 serving, as a suspension of a proof mass. The structure contains a frame 12, a proof mass 16 and a suspension 14 that connects the frame 12 and the proof mass 16. The proof mass 16 is characterized by its three dimensions: length 71, width 72, and thickness 73. Similarly, the suspension 14 is characterized by its three dimensions: length 74, width 75, and thickness 76. In general, thickness of the frame 77 can be different from the thickness 73 of the proof mass 16. The structure is formed in a silicon wafer using deep etching from the backside of the wafer, as it is shown in FIG. 1.
Center of gravity 9 of the proof mass 16 is located below the neutral plane of the suspension 14. Being loaded with vertical (Z) acceleration (perpendicular to the front surface of the chip), the proof mass 16 moves vertically following the direction of the force of inertia (FOI) 11. At a lateral acceleration (X or Y), parallel to the surface of the chip, the proof mass moves in rocking mode. One side of the proof mass 16 tends to move up and its other side tends to move down. In general case, as it shown in FIG. 1, when FOI 11 is applied in a random direction, the motion of center of gravity has all three components x, y, z in the coordinate system X, Y, Z.
Vertical acceleration creates stresses of the same sign along the periphery of the diaphragm. Lateral acceleration creates stresses of different sign along the periphery of the diaphragm near the frame and adjacent to the areas where the proof mass is coupled with the diaphragm. Stress distribution in the diaphragm depends on the direction of acceleration vector and this stress distribution is unique for each combination of direction and magnitude of acceleration vector.
Stress sensors 1, 2, 3, 4, 5, 6, 7, 8, located in eight local areas on the diaphragm, sense the stress created by a force. Being properly located in different places on the diaphragm, stress sensors provide signals representative of the local stress sensed, which allow measuring all three components of acceleration vector.
If other than a diaphragm type of suspension is used then the vertical acceleration will create stresses of the same sign in the respective areas of suspension. Lateral components of acceleration create stresses of different magnitudes in local positions of the suspension.
As a result, lateral components of acceleration vector can be detected, for example, using a differential signal from the sensors and vertical component of acceleration vector can be detected using sum of the signals from some of the sensors or all sensors.
The 3D accelerometers based on micromachined silicon chips with piezoresistors on the elastic element, suspension, for example flexible diaphragm or beams are known. Prior art, shown in FIGS. 1 and 2(a)-(b), uses a sensor chip 10 with a rigid frame 12 and a proof mass 16 of a die, connected with the frame by a thinner elastic element 14. In FIG. 1 this elastic element is a diaphragm. In FIG. 2(a) and FIG. 2(b) the elastic element is a combination of beams 90, 92, 94, 96.
Fabrication of 3D accelerometer die described in U.S. Pat. No. 5,485,749 and shown in FIG. 2b requires a non-standard initial material—silicon-on-insulator (SOI) wafers with buried cavities 80 formed in the handle wafer 82 below the suspension beams 14 before bonding handle wafer 82 and device wafer. After thinning of the device wafer the device layer 84 is used for suspension beams 14.
Use of non-standard initial material is undesirable in high-volume production for reasons including: high cost of initial material, additional processing steps in fabrication, limited number of suppliers, and potentially lower quality than standard initial materials.
FIG. 3(a)-(b) illustrates the examples of prior art piezoresisrors layout on the surface of elastic elements of three-axis accelerometers.
The piezoresistors 1, 3, 5, 7 in FIG. 1 are located at the periphery of the diaphragm adjacent to the frame 12, while piezoresistors 2, 4, 6, 8 are adjacent to the proof mass 16 and proper corresponding resistors are electrically connected into three Wheatstone bridges accommodating X, Y and Z components of an applied inertia force vector.
The disadvantages of these designs can be summarized as having: large numbers of stress sensitive components, poor long-term stability due to the metal interconnections on the surface of the suspension for the bridges, limited mechanical overload protection, large fluctuations in X, Y, Z sensitivities, large cross-axis sensitivity, no process integration with other sensors and CMOS, no scaling down in size and cost without compromising its performance.